As is known in the art, vehicles with a four wheel drive system are designed to operate under conditions in which maximum pulling force is required; under such conditions, the driver engages the gear box in order to transmit the power from the engine to the second drive shaft, normally the front one.
During the operation with two wheels (over 90% of the vehicle life), front drive is disconnected, at the gear box; but the front shaft remains always turning, driven by the wheels themselves. This causes a "drag" effect which, besides causing unnecessary wearing of the front shaft, increases fuel consumption and the wearing of the tires, and creates noises and vibrations, which may be harmful for the vehicle's driving.
In order to minimize such inconveniences, engaging/releasing mechanical devices are known, consisting of a shaft which is notched on one of the sides of the front differential box. The shaft remains released when the vehicle is on two wheel drive (4.times.2) and is engaged by a sliding element, controlled by the drive, when it is necessary to operate the vehicle under four wheel transmission mode (4.times.4).
Such devices do not solve the above mentioned problems, for they offer only a partial immobilization of the front drive system; even when the vehicle is being operated under two wheel drive mode (4.times.2), the front wheels drag a front shaft, part of the other shaft and the differential gears, with only the trunnion shaft not turning. In other words, a large number of parts remains connected to the wheels and, as such, the parts remain "dragged", even when the vehicle is operated under the 4.times.2 system.
In addition, the differential housing has to be specially designed, cast and machined, in order to house the device, with a resulting increase in costs of the original equipment, making its application totally prohibitive in the parts market.
As a solution for the above mentioned inconveniences, engaging systems known in the market as "free-wheel" have been used, which are coupled usually to each of the front wheels. These systems permit the drive to release the wheels from the drive shaft, whenever desired; in vehicles with such devices, while operating with two wheels only (4.times.2), the wheels are the only parts of the front driving system which remain turning in the same manner as in any other conventional vehicle with two wheel drive system, eliminating, therefore, unnecessary wear of the front driving system and cutting down fuel consumption.
Among the free-wheel devices known in the art, there are the hand driven and the automatic drive devices. The first one engages and releases the wheels to and from the drive shaft through different mechanical means. Although they reach their target in a satisfactory way, they are inconvenient, for they force the driver to get out of the vehicle's cabin whenever four wheel drive becomes necessary and, again, when such a system is no longer necessary.
The automatic devices used in the market do not force the driver to leave the cabin in order to engage/release the wheels, but have other inconveniences, such as a certain complex construction and the need to stop the vehicle and invert its direction, when releasing the wheels; this creates very difficult situations (such as front and back swing, steep slopes, etc.) and creates a bad operation of the transmission systems, with loss of power in the four wheels in critical moments, and untimely wear.
As an improved solution, when comparing with conventional manual and automatic devices, there are other devices in which the engaging of the wheels to the respective drive shafts is performed by a pneumatic, hydraulic or vacuum system, controlled by the driver through a command valve located within the vehicle's cabin, with such a valve forcing an unbalance of pressure within the device and moving the engaging/releasing element in order to either engage or release the wheels to and from the drive shafts, following the driving needs (with two or four wheels).
Said devices eliminate the inconveniences of the manual and automatic devices, and show several advantages, such as: operation within the cabin; they maintain the use of engine braking capacity; it is possible to release the wheels without changing the direction of the vehicle; the front shaft remains immobilized under 4.times.2 drive; trustworthy; simple construction and installation.
However, such devices still have some inconveniences, which shall be described hereinbelow, classified in the two possible systems: vacuum and pressurized (hydraulic or pneumatic).
In the vacuum system, while operating the vehicle under four wheel mode, the system is kept constantly connected, or under "depression", in such a way that the engaging/releasing device itself and the elements of the vehicle drive shaft are liable to attract, by aspiration, due to the "depression", solid and liquid particles which are close to them. Considering that most vehicles using four wheel drive operate in unfavorable areas (mud, dust, sand, etc.), the retainers located on the drive shaft and on the device, which have a purpose to seal the whole system, are continuously "bombed" by such particles, with heavy wear and short useful life, requiring constant replacements.
In the pressurized (hydraulic or pneumatic) system, constant pressurization of the system when the vehicle is operated under 4.times.4 mode, has the inconvenience of possible general leakage, derived from the wear of the different components caused by friction and by the fact that they are subject to variable temperatures, which generate deformations.